Photorefractive limiting quadratic processor

Abstract

Limiting quadratic processing and compansion in photorefractive two beam coupling is disclosed. Two-beam coupling in photorefractive barium titanate employs the imaged intensity of the signal to amplify the reference beam while maintaining the phase of the reference beam. The phase distorted signal beam is converted to that of the controlled phase of the reference beam. The high pump limit of amplification in this two-beam coupling device produces an amplitude compressed output to reduce multiplicative noise. Lost contrast of the image is thereafter restored. Beam clean-up of a non-intelligence bearing beam can be carried out by a similar process; a low pass filter consisting of a pinhole plate can be used in place of the second photorefractive crystal in the Fourier plane and only the planar wavefront portion will pass through the pinhole and may be collimated by a lens to provide a cleaned planar output beam.

What is claimed is: 
     
       1. A method of reducing multiplicative noise in an optical image comprising the steps of: (a) directing a noisy signal beam having an optical image thereon corrupted by complex multiplicative noise at a first nonlinear optical device;   (b) directing a phase restoring reference beam, coherent with respect to the noisy signal beam, into said first nonlinear optical device for generating nonlinear coupling therein which produces a phase restored output beam having an intensity which is less than the intensity of the noisy signal beam;   (c) Fourier transforming said phase restored output beam from said first nonlinear optical device and directing the resulting transformed phase restored output beam into a second nonlinear optical device; and   (d) directing a coherent noise reducing reference beam into said second nonlinear optical device for producing two beam coupling therein and a noise reduced output signal therefrom.   
     
     
       2. The method of claim 1 further including the step of directing a contrast restoring coherent reference beam into a third nonlinear optical device along with a Fourier transform of said noise reduced output signal for producing two beam coupling therein and a resulting contrast restored output beam. 
     
     
       3. The method of claim 2 including the step of causing the intensity of the noisy signal beam to be 100-1000 times greater than the intensity of the phase restoring reference beam. 
     
     
       4. The method of claim 3 wherein the intensity of the contrast restoring reference beam is two to ten times greater than the intensity of said noise reduced output signal bearing said optical image. 
     
     
       5. The method of claim 2 wherein the intensity of the contrast restoring reference beam is two to ten times greater than the intensity of the noise reduced output signal bearing said optical image. 
     
     
       6. Method of claim 2 wherein at least some of said nonlinear optical devices include photorefractive crystals. 
     
     
       7. The method of claim 1 including the step of causing the intensity of the noisy signal beam to be 100-1000 times greater than the intensity of the phase restoring reference beam. 
     
     
       8. Method of claim 7 wherein at least some of said nonlinear optical devices include photorefractive crystals. 
     
     
       9. Method of claim 1 wherein at least some of said nonlinear optical devices include photorefractive crystals. 
     
     
       10. A method of reducing multiplicative noise in an optical image comprising the steps of: (a) inserting a noisy signal beam having an optical image thereon corrupted by complex multiplicative noise into a first nonlinear optical device;   (b) directing a phase restoring non-image bearing reference beam, coherent with respect to the noisy signal beam, into said first nonlinear optical device for generating nonlinear coupling therein which produces a phase restored output beam; and   (c) causing the intensity of the noisy signal beam to be 100-1000 times greater than the intensity of the phase restoring reference beam.